1. Field of the Invention
The present invention relates to a focus detecting device for use in a camera and an image signal amplifier circuit for such a focus detecting device.
2. Discussion of Background
Some focus detecting devices for cameras employ a solid-state self-scanned image sensor such as a charge-coupled device (CCD) for amplifying an output signal thereof. In such focus detecting devices, a signal representing a black reference is produced prior to an image signal of the charge-coupled device. This black reference signal is generated by covering a few bits close to the output end of the photodiode array of the charge-coupled device with an aluminum electrode and using a signal from the covered area as a signal of the darkest portion of the output signal from the charge-coupled device. The effective output signal from the charge-coupled device can be obtained by amplifying the difference between the black reference signal and the next image signal sent from the charge-coupled device.
According to the above process, however, even a signal indicative of the dark portion of an object is considerably larger than the black reference level as indicated by the output signal from a charge-coupled device in FIG. 8 of the accompanying drawings. Where the output signal is converted to a digital signal and the digital signal is processed for focus detection in a camera, the voltage range represented by a in FIG. 8 is wasted. In order to increase the accuracy of the focus detection, it is necessary to make the degree of signal amplification as large as possible. If the signal shown in FIG. 8 were amplified, then the wasteful range a would also be increased, and a portion of the signal would extend beyond the input range of the analog-to-digital (A/D) converter.
As is known from Japanese Laid-Open Patent Publication No. 60(1985)-101516, a focus detecting device in a camera operates by amplifying and processing the image signal from a charge-coupled device for focus detection. When the integration (charge storage) time of the charge-coupled device becomes longer, a monitor signal from a monitor circuit in the charge-coupled device is compared with a reference voltage, and the degree of amplification for the image signal is varied in several steps according to the result of the degree of comparison. For example, the amplification is increased by the factor of 2, i.e., into degree of amplification that are twice, four times, and eight times larger; however, there are no intermediate degrees of amplification degrees between these increased discrete amplification degrees.
As described above, the degree amplification degree for the image signal in the focus detecting device is varied in a step-like discrete manner. Therefore, for an object which has a slightly low contrast and which has a brightness that the degree of amplification for the image signal is twice as large, the focus detection can properly be effected and the camera can be focused through focus adjustment. However, when an object is slightly brighter and the amplification degree is not increased, the amplification degree is not high enough to obtain proper focus detection, thereby resulting in a focusing failure.
Illumination of an object for a focus detecting device employing a charge-storage-type light detector is known from Japanese Laid-Open Patent Publication Nos. 57(1982)-105710, 60(1985)-46513, and 60(1985)-46514, for example.
According to Japanese Laid-Open Patent Publication No. 57(1982)-105710, however, it takes time to pick up the output from the light detector for the purpose of illuminating the object. With the arrangements of Japanese Laid-Open Patent Publication Nos. 60(1985)-46513 and 60(1985)-46514, it is necessary to measure the distance up to the object so as to determine whether the object is to be illuminated, and hence the focus detecting process is also time-consuming.